Flexible printed circuits have gained widespread use in applications where flexing is a functional requirement, such as where flexing facilitates assembly and disassembly, or other dimensional variations need to be accommodated. In general, flexible circuits are formed by bonding a conductive layer, e.g., a metal foil as copper, onto a flexible insulating film or substrate such as a polyester or polyimide resin film. The substrate is generally temperature-resistant and dimensionally stable. A circuit is formed in the metal foil by conventional etching techniques.
Flexible printed circuits may be covered with an additional layer of a similar insulating film or substrate and bonded with an adhesive to provide additional insulation. The additional layer of the insulating film may also protect the circuit pattern from contamination, corrosion, and mechanical damage. Further, the cover film may also be necessary for the formation of a symmetrical layered structure and/or a solder stop mask.
A problem associated with flexible printed circuit boards is that heat generated by a current flowing through the circuit can cause the substrate, the cover film, or the adhesive to be ignited. Thus, a flexible printed circuit board is a potential cause of fire in instruments and devices utilizing such boards. One approach to this problem is to include flame retardants in an adhesive applied to the board. Generally, the adhesive may be rendered flame-resistant or self-extinguishing by adding halogenated flame inhibitors.
EP-B-14-444 is directed to an insulating laminate which consists of several sheets of reinforcements impregnated with a resin and layers of resin between each reinforcement. According to EP-B-14-444, a curable resin is dissolved in solvent and used to saturate the reinforcement sheets. A copper foil is applied to the saturated reinforcement sheets, and laminated under pressure and heat.